This invention relates to fixation screws, and more particularly to fixation screws which are used to secure bone/ligament or tendon grafts in place in boney tunnels throughout the body.
Ligaments are connective tissue which join surfaces of bones together in a joint. They act to limit the motion of the bones of the joint relative to each other. Injuries to ligaments are not uncommon, particularly in patients who are active in sports. The anterior cruciate ligament of the human knee is especially susceptible to damage. Unfortunately, when an anterior cruciate ligament is damaged, it must often be replaced because it frequently never heals.
One method of replacing damaged ligaments is to use a section of tendon grafted from the knee cap or patella. A portion of tendon is excised from the patella. A portion of the bone (bone graft) to which each end of the tendon section is attached is excised with the tendon section. A hole or boney tunnel (ligament tunnel) is drilled through the femur and tibia. The tendon graft is inserted into the ligament tunnel in the femur and the ligament tunnel in the tibia and positioned so that it is centered in the two ligament tunnels. That is, an equal length of tendon graft is disposed in both the femur and in the tibia. A fixation screw is then tightened in the ligament tunnel between the bone graft and the side of the ligament tunnel in the bone to affix the bone graft in place. The tendon section is now appropriately tensioned and the bone graft attached to the other end of the tendon section is secured with a screw.
Heretofore, metal fixation screws have been used to affix the bone grafts in place. Such screws may be designed to be interference screws or may be standard bone screws with blunt threads. The interference screw is designed so that the screw will not cut into the bone graft or the side of the boney tunnel. Rather, when the interference screw is tightened in place, it forces the bone graft tightly against the side of the boney tunnel so that the bone graft is held in place by friction.
Metal screws do not always act as interference screws, even if designed as such. The threads of the metal screws can cut into the bone grafts and damage or even destroy the bone grafts. Further, if either the screw or the bone graft is slightly oversize, or the boney tunnel in which the bone graft is to be secured is slightly undersized, the lack of resiliency on the part of the threads of the metal screw can cause too much force to be exerted on the bone graft as the metal screw is tightened. This too can damage or destroy the bone graft. If this happens, the surgeon may be left without a bone graft to use to secure an end of the tendon graft in place. The tendon graft may then be useless. If this happens, either a new tendon graft must be taken from the patella or, if there is not sufficient tendon left in the patella to take a new graft, a different technique for replacing the damaged ligaments must be used such as an artificial ligament prosthesis. In most cases, however, using a natural part of the patient's body to replace the damaged ligament is preferable. Artificial ligaments have biocompatibility and biodegradation problems which a tendon section from the patient's own patella does not have.
Another problem with the metal screws is that the length of tendon which can be grafted varies between individual patients. So too do the lengths of the ligament tunnels drilled in the tibia and femur. Consequently, when the tendon grafts are properly positioned in the ligament tunnels drilled in the femurs and tibias of patients, the distances the bone grafts are recessed in the ligament tunnels in the femurs and tibias vary from patient to patient. The screw which is used to affix the bone graft in place must be long enough to have adequate purchase against the bone graft but short enough so that any portion extending beyond the surface of the tibia or femur when the screw is tightened is minimized and preferably eliminated. Therefore, the surgeon must have available screws in several different lengths to be able to select ones having the proper length.
Finally, metal screws require separate instrumentation. In this context, "instrumentation" means a device which is used to facilitate the installation of the screw. Here, such instrumentation is some type of fastening device, such as a hex driver, which is used to tighten the screws. This requires that sterile instrumentation (fastening device) be available to the surgeon. This increases the number of sterile items that must be maintained in the operating room.
It is an object of this invention to provide a fixation screw for securing in place in a ligament tunnel bone grafts of a tendon graft used to replace ligaments. The fixation screw is formed as an interference screw from a relatively soft material to prevent the threads of the screw from cutting into and damaging the bone grafts.
It is another object of this invention to provide a fixation screw where any excess projecting beyond the surface of the bone at which the ligament tunnel opens can be trimmed off after the fixation screw has been tightened.
It is yet another object of this invention to provide a fixation screw which has integral instrumentation used to tighten the screw in place and which is cut off when the excess screw material is trimmed after the screw has been tightened.